Standard Test Methods for Fluoride Ion in Water 1

Transcription

1 Designation: D An American National Standard Standard Test Methods for Fluoride Ion in Water 1 This standard is issued under the fixed designation D 1179; the number immediately following the designation indicates the year of original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A superscript epsilon (e) indicates an editorial change since the last revision or reapproval. This standard has been approved for use by agencies of the Department of Defense. 1. Scope 1.1 These test methods 2 cover the determination of fluoride ion in water. The following two test methods are given: Sections Test Method A Distillation 7 to 13 Test Method B Ion Selective Electrode 14 to Test Method A covers the accurate measurement of total fluoride in water through isolation of the fluoride by distillation and subsequent measurement in the distillate by use of the ion selective electrode (ISE) method. The procedure covers the range from 0.1 to 2.6 of fluoride. 1.3 Test Method B covers the accurate measurement of simple fluoride ion in water by means of an ion selective electrode. With this test method, distillation is eliminated because the electrode is not affected by the interferences common to colorimetric procedures. Concentrations of fluoride from 0.1 to 1000 may be measured. 1.4 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use. For a specific precautionary statement, see Former Test Method A, SPADNS Photometric Procedure, was discontinued. Refer to Appendix X1 for historical information. 2. Referenced Documents 2.1 ASTM Standards: D 1066 Practice for Sampling Steam 3 D 1129 Terminology Relating to Water 3 D 1192 Specification for Equipment for Sampling Water and Steam in Closed Conduits 3 D 1193 Specification for Reagent Water 3 1 These test methods are under the jurisdiction of ASTM Committee D-19 on Water and are the direct responsibility of Subcommittee D19.05 on Inorganic Constituents in Water. Current edition approved Feb. 10, Published April Originally published as D Last previous edition D Bellack, E., Simplified Fluoride Distillation Method, Journal of the American Water Works Association, Vol 50, 1958, p Annual Book of ASTM Standards, Vol D 2777 Practice for Determination of Precision and Bias of Applicable Methods of Committee D-19 on Water 3 D 3370 Practices for Sampling Water from Closed Conduits 3 3. Terminology 3.1 Definitions For definitions of terms used in these test methods, refer to Terminology D Significance and Use 4.1 Simple and complex fluoride ions are found in natural waters. Fluoride forms complexing ions with silicon, aluminum, and boron. These complexes may originate from the use of fluorine compounds by industry. 4.2 Fluoridation of drinking water to prevent dental caries is practiced by a large number of communities in this country. Fluoride is monitored to assure that an optimum treatment level of 1.4 to 2.4, depending on the corresponding range of ambient temperatures of 32 to 10 C, is maintained. 5. Purity of Reagents 5.1 Reagent grade chemicals shall be used in all tests. Unless otherwise indicated, it is intended that all reagents shall conform to the specifications of the Committee on Analytical Reagents of the American Chemical Society, where such specifications are available. 4 Other grades may be used, provided it is first ascertained that the reagent is of sufficiently high purity to permit its use without lessening the accuracy of the determination. 5.2 Purity of Water Unless otherwise indicated, references to water shall be understood to mean Type I reagent water conforming to Specification D Sampling 6.1 Collect the sample in accordance with Practice D 1066, Specification D 1192, or Practices D 3370, as applicable. 4 Reagent Chemicals, American Chemical Society Specifications, American Chemical Society, Washington, DC. For suggestions on the testing of reagents not listed by the American Chemical Society, see Analar Standards for Laboratory Chemicals, BDH Ltd., Poole, Dorset, U.K., and the United States Pharmacopeia and National Formulary, U.S. Pharmaceutical Convention, Inc. (USPC), Rockville, MD. Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA , United States. 1

2 TEST METHOD A DISTILLATION 7. Scope 7.1 This test method is applicable to the accurate determination of fluoride ion in water, including most wastewaters. This test method may not be applicable to concentrated brines and oily wastes. 7.2 This test method was tested on reagent water and wastewater. It is the user s responsibility to ensure the validity of this test method for waters of untested matrices. 8. Summary of Test Method 8.1 The fluoride is distilled as hydrofluosilicic acid and is determined by the ion selective electrode. 9. Interferences 9.1 In sample distillation, interferences may be experienced due to the following factors Aluminum in excess of 300 and silicon dioxide as colloidal silica in excess of 400 will hold up in the condenser to a certain extent, causing low results and acting as a positive interference for subsequent samples of lower fluoride content. In these cases, the condenser should be flushed with 300 to 400 ml of water and the washwater added to the distillate. The distillate may then be diluted to 1.0 L. If the analyst prefers, a smaller sample aliquot diluted to 300 ml may be selected for distillation Sea water, brines, and generally samples of dissolved solids in excess of 2500 will cause bumping in the distillation flask. Dilution of the sample with fluoride-free water to a lesser-dissolved solids concentration is an effective remedy to bumping Samples containing oily matter which may result in a two-phase distillate, an emulsion, or anything other than a clear distillate may prevent accurate measurement of fluoride. Such samples should be extracted initially with a suitable solvent (such as ether, chloroform, benzene, and similar solvents) to remove the oily material, and then warmed on a steam bath to remove traces of the added solvent. 10. Apparatus 10.1 Distillation Assembly Glassware consisting of a 1-L, round bottom, borosilicate boiling flask, an adapter with a thermometer opening, a connecting tube, a condenser, and a thermometer reading to 200 C, assembled as shown in Fig. 1. Standard-taper or spherical ground glass joints shall be used throughout the apparatus. 11. Reagents 11.1 Silver Sulfate (Ag 2 SO 4 ), powder Sodium Arsenite Solution (2 g/l) Dissolve 2gof sodium arsenite (NaAsO 2 ) in water and dilute to 1 L Sodium Fluoride Solution, Standard (1.0 ml = 0.01 mg F) Dissolve g of sodium fluoride (NaF) in water and dilute to 1.0 L. Dilute 100 ml of this solution to 1.0 L with water. Store in borosilicate glass or polyethylene Sulfuric Acid (H 2 SO 4 ), Concentrated (sp gr 1:84). 12. Procedure 12.1 Distillation: A Heating mantle (quartz). B Round-bottom flask, 1000 ml. C Adapter with thermometer opening. D Thermometer, 200 C. E Connecting tube. F Graham condenser, 300 mm. G Vessel, calibrated at 300 ml. FIG. 1 Distillation Assembly for Fluoride Isolation Place 400 ml of water in the distilling flask and add 200 ml of concentrated H 2 SO 4 (sp gr 1.84). Observe the usual precautions while mixing the H 2 SO 4 by slow addition of the acid accompanied by constant swirling. Add sufficient boiling stones 5 and assemble the apparatus as shown in Fig. 1. Heat the solution in the flask, preferably with an electric heating mantle, until the temperature of the contents reaches exactly 180 C. (A quartz heating mantle is preferred in order to reach the required 180 C in a minimum time.) While heating, the tip of the thermometer must extend below the level of the liquid in the flask. Discard the distillate. The procedure, to this step, serves to adjust the acid-water ratio for subsequent distillations Caution Cool the acid-water mixture to below 100 C, slowly add 300 ml of sample, and mix thoroughly before heating. Distill as described in , until the temperature reaches 180 C. Do not allow the temperature to exceed 180 C; excessive carryover of sulfate occurs at temperatures of or above 180 C, resulting with interference in the subsequent fluoride measurement Collect the distillate in any suitably calibrated vessel. If a calibrated vessel is used, it is possible to dispense with thermometer readings and stop the distillation when the volume of distillate reaches 300 ml In the case of samples containing chlorides in concentrations which may interfere in the subsequent reaction, add Ag 2 SO 4 to the distillation mixture at a rate of 5 mg/mg of chloride. 5 Glass beads must be of a soft glass (rather than borosilicate). Use about 12 beads. Soft beads will provide silica to the fluoride and protect the distillation flask. 2

3 TABLE 1 Determination of Precision Final Statistics for Test Method A NOTE 1 Precision of Test Method A was determined from round robin data using distillation with ion selective electrode finish. Amount added, Reagent Water Concentration, x A S t A S o A Wastewater Concentration, x A A S t A S o A A Calculated with outlier point removed from data base The acid-water distilling solution may be used repeatedly until the buildup of interference materials equals the concentration given in Section Analysis: Use Test Method B (Ion Selective Electrode) with the buffer solution described in Precision and Bias 13.1 The precision and bias for Test Method A, shown in Table 1 and Table 2 were determined using distillation followed by an ion selective electrode finish. Four concentrations and three replicates were provided by six laboratories for reagent water and wastewater. TEST METHOD B ION SELECTIVE ELECTRODE 14. Scope 14.1 This test method is applicable to the measurement of fluoride ion in finished waters, natural waters, and most industrial wastewaters. With this test method, distillation is eliminated and concentrations of fluoride from 0.1 to 1000 may be measured The test method is not applicable to samples containing more than of dissolved solids This test method was tested on reagent water. It is the user s responsibility to ensure the validity of this test method for waters of untested matricies. TABLE 2 Determination of Bias for Test Method A NOTE 1 Bias of Test Method A was determined from round robin data using distillation with ion selective electrode finish. Amount added Mean recovery Bias % Bias Statistically significant at 5 % level Reagent Water % no % no % yes % yes Wastewater % no % yes % yes % yes TABLE 3 Allowable Interference Levels with Selective Ion Electrode and Buffer A Interfering Ion Maximum Allowable Concentration at 1.0 F Al Si Fe A Refer to 16.2 for description of interfering cations. 15. Summary of Test Method 15.1 The fluoride is determined potentiometrically using an ion selective fluoride electrode in conjunction with a standard single junction, sleeve-type reference electrode, and a ph meter having an expanded millivolt scale, or an ISE meter having a direct concentration scale for fluoride The fluoride electrode consists of a lanthanum fluoride crystal across which a potential is developed by fluoride ions. 6,7 The cell may be represented by Ag/AgCl, Cl (0.3), F (0.001) LaF 3 /test solution/kcl/agcl/ag. 16. Interferences 16.1 Extremes of ph interfere; sample ph should be between 5.0 and Polyvalent cations of Si +4,Fe +3, and Al +3 interfere by forming complexes with fluoride. The degree of interference depends upon the concentration of fluoride, and the ph of the sample (see Table 3). The addition of the buffer solution, 18.1, buffers the solution ph and will complex small amounts of aluminum, as well as silicon and iron. The use of one of the two selective buffers (see 18.2 and 18.3) is recommended when aluminum is present, because they are more effective over a greater range of aluminum concentrations Interferences usually encountered in other test methods, such as sulfate or phosphate, do not affect this test method. 17. Apparatus 17.1 ph Meter, with expanded millivolt scale or ISE meter Fluoride Ion Selective Electrode Reference Electrode, single-junction sleeve-type Mixer, magnetic, with a TFE-fluorocarbon coated stirring bar. 18. Reagents 18.1 Buffer Solution (ph from 5.0 to 5.5) To approximately 500 ml of water, add 57 ml of glacial acetic acid (sp gr 1.06), 58 g of sodium chloride (NaCl), and 0.30 g of sodium citrate dihydrate in a 1000-mL beaker. Stir the solution to dissolve, and cool to room temperature. Adjust the ph of the solution to between 5.0 and 5.5 with 5 N sodium hydroxide (NaOH) (about 150 ml will be required). Transfer the solution to a 1000-mL volumetric flask and dilute to the mark with water. 6 Frant, M.S., and Ross, J.W., Electrode for Sensing Fluoride Ion Activity in Solution, Science, Vol 154, 1966, p Frant, M.S., and Ross, J.W., Use of a Total Ionic Strength Adjustment Buffer for Electrode Determination of Fluoride in Water Supplies, Anal. Chem., Vol 40, 1968, p

4 TABLE 4 Determination of Precision Without Distillation using Buffers Concentration, Buffer A Buffer B S t S o S t S o Buffer A for Aluminum To approximately 500 ml of water, add 84 ml of reagent grade hydrochloric acid (sp gr 1.19), 242 g of tris-(hydroxymethyl)-aminomethane (THAM) (also known as 2-amino-2-(hydroxymethyl)-1,3- propanediol), and 230 g of sodium tartrate (Na 2 C 4 H 4 O 6 2H 2 O). Stir to dissolve, and cool to room temperature. Transfer the solution to a 1000-mL volumetric flask and dilute to the mark with water Buffer B for Aluminum Dissolve 60 g of citric acid monohydrate, 210 g of sodium citrate dihydrate, and 53.5 g of ammonium chloride in 500 ml of water. Add 67 ml of ammonium hydroxide (sp gr 0.90). Transfer the solution to a 1000-mL volumetric flask and dilute to the mark with water Sodium Fluoride Solution, Standard (1.0 ml = 0.01 mg of F) See Calibration 19.1 Prepare a series of three standards, 0.5, 1.0, and 2.0, using the standard fluoride solution (see 18.4). Dilute the following volumes to 100 ml: Fluoride Solution, Standard, ml (1.0 ml = 0.01 mg F) Concentration, mg F/L For unusual waters containing high concentrations of fluorides, the range of standards may be expanded up to 1000, if necessary Pipet 50 ml of each standard into a 150-mL beaker. Using a pipet, add 50 ml of buffer. Mix each solution well using a magnetic stirrer Calibration of ph Meter Immerse the ph electrodes in each standard solution, starting with the lowest concentration, and measure the developed potential while mixing. The electrodes must remain in the solution for at least 3 min, or until the reading has stabilized. Using semilogarithmic graph paper, plot the concentration of fluoride in milligrams per litre on the logarithmic axis versus the electrode potential developed in the standard on the linear axis, starting with the lowest concentration at the bottom of the scale Calibration of ISE Meter Follow the directions of the manufacturer for the operation of the instrument. See 19.1 and for selection of standards. 20. Procedure 20.1 Place 50.0 ml of the sample and 50.0 ml of buffer into a 150-mL beaker. Place the solution on a magnetic stirrer and mix at medium speed. Immerse the electrodes in the solution and observe the meter reading while mixing. The electrodes TABLE 5 Determination of Bias Without Distillation (Buffer A) Amount Added, Amount Found, 6 Bias 6 % Bias TABLE 6 Determination of Bias Without Distillation (Buffer B) Amount Added, Amount Found, 6 Bias 6 % Bias must remain in the solution for at least 3 min, or until the reading has stabilized. At concentrations less than 0.5 of fluoride, it may take as long as 5 min to reach a stable meter reading; higher concentrations stabilize more quickly. If a ph meter is used, record the potential measurement for each unknown sample and convert the potential reading to the fluoride ion concentration of the unknown, using the standard curve. If an ISE meter is used, read the fluoride level in the unknown sample directly in milligrams per litre on the fluoride scale. NOTE 1 It is recommended that the solution samples are the same temperature as calibration standards. This recommendation was not in place during the original round robin. 21. Calculation 21.1 Calculate the concentration of fluoride ion, in milligrams per litre, in accordance with Eq 1 Fluoride, 5 50A/V (1) where: A = fluoride measured by using the Test Method B (Ion Selective Electrode) and the buffer solution described in 18.1, and V = sample, ml. 22. Precision and Bias The precision of this test method, using the buffer solution described in 18.1 (see Table 4), was tested by 111 laboratories doing single analysis and found to be at a concentration of 0.85 of fluoride The precision of this test method (Table 5), using the buffer solution A described in 18.2, was tested by seven operators in five laboratories doing three replicates of four concentrations. Each concentration was analyzed in the presence of 20 of aluminum. The results were from analysis in reagent waters. It is the responsibility of the user to test the application to other matrices The precision of this test method (Table 6), using the buffer solution B described in 18.3 was tested by six operators 8 Supporting data are available from ASTM Headquarters. Request RR:D

5 in four laboratories doing three replicates of four concentrations. Each concentration was analyzed in the presence of 20 of aluminum. The results were from the analysis of reagent water. It is the responsibility of the user to test the application to other matrices This precision and bias statement conforms to Practice D , which was in place at the time of round robin testing. It does not meet the requirements of Practices D Keywords 23.1 analysis; electrode; fluoride; water APPENDIX (Nonmandatory Information) X1. RATIONALE FOR DISCONTINUATION OF TEST METHODS X1.1 SPADNS Procedure, Photometric X1.1.1 This test method was discontinued in The test method may be found in its entirety in the 1988 Annual Book of ASTM Standards, Vol X1.1.2 This test method is applicable to the accurate determination of fluoride ion in water, including most wastewaters. This test method may not be applicable to certain exceptions, such as concentrated brines and oily wastes. X1.1.3 The fluoride is distilled as hydrofluosilicic acid and is determined photometrically by its bleaching effect upon the SPADNS dye. Distillation may be omitted without loss of precision only if it is known that the concentration of certain interfering substances will not cause a 0.1 error at 1.0 mg F/L. X1.1.4 This test method was discontinued and replaced by a distillation method which determines fluoride content in water by the selective ion electrode. The SPADNS photometric procedure was discontinued because there were insufficient laboratories interested in participating in another collaborative study to obtain the necessary precision and bias as required by Practice D ASTM International takes no position respecting the validity of any patent rights asserted in connection with any item mentioned in this standard. Users of this standard are expressly advised that determination of the validity of any such patent rights, and the risk of infringement of such rights, are entirely their own responsibility. This standard is subject to revision at any time by the responsible technical committee and must be reviewed every five years and if not revised, either reapproved or withdrawn. Your comments are invited either for revision of this standard or for additional standards and should be addressed to ASTM International Headquarters. Your comments will receive careful consideration at a meeting of the responsible technical committee, which you may attend. If you feel that your comments have not received a fair hearing you should make your views known to the ASTM Committee on Standards, at the address shown below. This standard is copyrighted by ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA , United States. Individual reprints (single or multiple copies) of this standard may be obtained by contacting ASTM at the above address or at (phone), (fax), or service@astm.org ( ); or through the ASTM website ( 5